Key Topics

• Atmosphere
• Lithosphere
• Hydrosphere
• Biosphere
• Earth’s Interior Structure

Composition of the Earth

The four components of the earth, Source: Jain, S. (2014)

Interrelationship of Earth components

The four components of the earth, Source: (a) https://byjus.com (b) Jain, S. (2014)

Atmosphere

Definition

• Atmosphere: A thin layer of gases surrounding Earth, essential for supporting life, regulating temperature, and enabling weather patterns.

Composition of the Atmosphere

• Major Components:
  • Nitrogen (N2): 78%
  • Oxygen (O2): 21%
  • Trace Gases: Argon, Carbon Dioxide, Neon, Helium, Methane, etc.
• Importance of Composition:
  • Nitrogen: Stabilizes the atmosphere, dilutes oxygen.
  • Oxygen: Essential for respiration, supports combustion.
  • Trace Gases: Play roles in greenhouse effect, climate regulation.

Layers of the Atmosphere

• Troposphere:
  • Closest to Earth, extends up to ~12 km.
  • Weather occurs here.
  • Temperature decreases with altitude.
• Stratosphere:
  • Extends from ~12 km to 50 km.
  • Contains the ozone layer, which absorbs UV radiation.
  • Temperature increases with altitude due to ozone absorption.
• Mesosphere:
  • Extends from ~50 km to 85 km.
  • Meteors burn up in this layer.
  • Temperature decreases with altitude.
• Thermosphere:
  • Extends from ~85 km to 600 km.
  • Temperature increases significantly.
  • Contains the ionosphere, where auroras occur.
• Exosphere:
  • Outermost layer, extends beyond 600 km.
  • Gradual transition into space.
  • Very low density of particles.

Layers of the Atmosphere, Source: https://byjus.com

Lithosphere

• Definition: The lithosphere is the land part of the Earth (litho = stone) and includes all the solid materials constituting the Earth, from surface downward.

• Structure:

  • Composed of both the crust and upper mantle.
  • Extends 80–100 km below the surface.
  • Thickest under continents (~120 km) and thinnest under mid-ocean ridges.

• Parts of the Plates:

  • Continental Lithosphere:
    • Thicker and less dense.
    • Composed primarily of granite.
  • Oceanic Lithosphere:
    • Thinner and more dense.
    • Composed primarily of basalt.

• Temperature:

  • Near the mantle surface: ~1,000°C.
  • Temperature increases by 25°C per 1,000 m as you move downward.
  • Reaches up to 3,700°C at the core-mantle boundary.

• Plate Tectonics:

  • The lithosphere is fragmented into massive plates.
  • These plates fit around the globe like pieces of a jigsaw puzzle.
  • Plates move independently and slide on the asthenosphere.
  • Movement causes earthquakes.

a- Tectonic. b- Chemical (Compositional) and Mechanical layers of the earth and their major characteristics., Source: https://byjus.com

Hydrosphere

• Definition:
  The hydrosphere encompasses all of Earth’s water, including water in liquid, solid, and vapor forms. It covers every part of the planet, from oceans and rivers to underground aquifers and atmospheric moisture.

• Coverage Percentage:

  • Total Earth Surface Covered by Water: ~71%
    • Oceans: 97% of Earth’s water, covering about 70% of the surface.
    • Freshwater: 3% of Earth’s water, found in glaciers, ice caps, rivers, lakes, and groundwater.

• Water Cycle:
  The water cycle describes the continuous movement of water on, above, and below the surface of the Earth. Key processes include:

  • Evaporation: Water from oceans, lakes, and rivers turns into vapor and rises into the atmosphere.
  • Condensation: Water vapor cools and forms clouds.
  • Precipitation: Water falls back to the Earth’s surface as rain, snow, sleet, or hail.
  • Infiltration: Water soaks into the ground, replenishing aquifers.
  • Runoff: Water flows over the land into bodies of water, completing the cycle.

• Significance:

  • Climate Regulation: Distributes heat around the globe, influencing weather and climate patterns.
  • Supporting Life: Essential for all living organisms, providing hydration and habitats.
  • Geological Processes: Shapes landscapes through erosion and sediment transport.

The Water Cycle, Source: https://www.twinkl.com.tr

Biosphere

• Definition:
  • The biosphere encompasses all zones on Earth where life is present, including the Lithosphere, Hydrosphere, and Atmosphere.
• Conditions for Life:
  • Life on Earth requires:
    • Water: Essential for all living organisms.
    • Source of Energy: Primarily sunlight, necessary for photosynthesis.
    • Nutrients: Found in soil, water, and air, required for growth and survival.
• Location of the Biosphere:
  • Exists within a narrow layer near the Earth’s surface.
  • Primarily within the upper layer of the Earth’s crust.
• Hierarchical Nature:
  • The biosphere is structured in a hierarchical manner, with various levels of organization from individual organisms to entire ecosystems.

Earth’s Interior Structure

• Layers of the Earth
  • Crust: Composition, continental vs. oceanic
  • Mantle: Structure, composition, convection currents
  • Core: Inner and outer core, composition, geodynamo

The Earth’s interior, Source: Jain, S. (2014)

Earth’s Interior Structure

• Crust:
  • Continental Crust:
    • Depth: ~35-70 km
    • Physical State: Solid
    • Major Components: Granite, silicate minerals
  • Oceanic Crust:
    • Depth: ~5-10 km
    • Physical State: Solid
    • Major Components: Basalt, gabbro
• Upper Mantle:
  • Depth: ~70-400 km
  • Physical State: Solid (partially plastic near the asthenosphere)
  • Major Components: Peridotite, olivine, pyroxene
• Transition Region:
  • Depth: ~410-650 km
  • Physical State: Solid
  • Major Components: High-pressure minerals (e.g., wadsleyite, ringwoodite)
• Lower Mantle:
  • Depth: ~660-2,890 km
  • Physical State: Solid
  • Major Components: Silicate minerals, magnesium, and iron oxides
• D” Layer:
  • Depth: ~2,700-2,900 km
  • Physical State: Solid (with some partial melting) it is often identified as part of the lower mantle, seismic data suggest that this layer might differ chemically from the lower mantle.
  • Major Components: Complex and variable, includes silicates and oxides
• Outer Core:
  • Depth: ~2,900-5,150 km
  • Physical State: Liquid
  • Major Components: Iron, nickel, sulfur
• Inner Core:
  • Depth: ~5,150-6,378 km
  • Physical State: Solid
  • Major Components: Iron, nickel

Understanding Earth Through Seismology

_ Body waves: they are transmitted through Earth materials. - P Waves (Primary Waves): - Definition: P waves are seismic waves that are the fastest and first to be detected. - What ‘P’ Stands For: Primary - Physical Properties: - Compressional waves that move in a push-pull motion. - Travel through solids, liquids, and gases. - Speed: ~6-14 km/s in the Earth’s crust. - Indications: - P waves can pass through the entire Earth, indicating that the outer core is liquid since they slow down and refract.

• S Waves (Secondary Waves):
  • Definition: S waves are slower than P waves and arrive after them.
  • What ‘S’ Stands For: Secondary
  • Physical Properties:
    • Shear waves that move perpendicular to the direction of travel (up and down or side to side).
    • Only travel through solids.
    • Speed: ~3-7 km/s in the Earth’s crust.
  • Indications:
    • The inability of S waves to travel through liquids confirms that the outer core is liquid, as S waves are not detected in this region.
• Significance:
  • The behavior of P and S waves provides crucial information about the Earth’s internal structure, including the existence and properties of the core, mantle, and crust.

A compression wave can be illustrated by a spring (like a Slinky) that is given a sharp push at one end. A shear wave can be illustrated by a rope that is given a quick flick., Source: Steven, E. (2015)

Velocity of waves

P wave (red) and S wave (blue) velocity variations with depth in Earth, Source: Steven, E. (2015)

Seismic Discontinuities

• Mohorovičić Discontinuity (Moho):
  • Definition: The boundary between the Earth’s crust and the mantle.
  • Depth:
    • Continental Crust: ~30-50 km
    • Oceanic Crust: ~5-10 km
  • Features:
    • Marked by a sudden increase in seismic wave velocities.
    • Indicates a transition from less dense rocks in the crust (granite or basalt) to denser rocks in the mantle (peridotite).
    • Key in understanding crustal thickness and composition.
• Gutenberg Discontinuity:
  • Definition: The boundary between the Earth’s mantle and the outer core.
  • Depth: ~2,900 km
  • Features:
    • Characterized by a sharp decrease in seismic wave velocities.
    • P waves slow down, and S waves disappear, indicating the transition from the solid mantle to the liquid outer core.
    • Crucial for understanding the Earth’s core composition and the dynamics of the Earth’s magnetic field.

The Temperature of Earth’s Interior

Right: generalized rate of temperature increase with depth within Earth.Left: Rate of temperature increase with depth in Earth’s upper 500 kilometres Source: Steven, E. (2015)

The Temperature of Earth’s Interior (2)

• Source of Heat:
  • Radioactive Decay: The decay of radioactive isotopes (e.g., uranium, thorium, and potassium) in the mantle and crust generates heat.
  • Residual Heat: Leftover heat from Earth’s formation, during the accretion and differentiation processes.
  • Core Formation: The release of latent heat as the Earth’s core solidifies contributes to the internal heat.
• Geothermal Gradient:
  • Definition: The rate of temperature increase with depth.
  • Average Gradient: ~25-30°C per kilometer in the crust.
  • Variability:
    • Tectonically Active Regions: Can exceed 40°C/km.
    • Stable Continental Interiors: Can be lower, around 15-20°C/km.
  • Mantle Gradient: Lower than the crust due to convective heat transfer, leading to a more uniform temperature distribution with depth.

Isostasy

Source: Steven, E. (2015)

• Definition:
  • Isostasy is the state of gravitational equilibrium between Earth’s lithosphere and asthenosphere.
  • The lithosphere “floats” on the denser, more fluid asthenosphere, similar to how icebergs float on water.
  • Variations in topography (e.g., mountains, valleys) are balanced by variations in the thickness and density of the lithosphere.
• Factors Impacting Isostasy:
  • Crustal Density:
    • Denser crust (e.g., oceanic crust) will sink deeper into the asthenosphere compared to less dense crust (e.g., continental crust).
  • Crustal Thickness:
    • Thicker crust (e.g., mountain ranges) results in greater isostatic adjustment and deeper sinking into the asthenosphere.
  • Erosion and Sedimentation:
    • Erosion reduces crustal weight, leading to isostatic rebound; sedimentation adds weight, causing subsidence.
  • Ice Loading and Melting:
    • Glacial ice adds weight, causing the crust to sink; melting ice results in rebound as the crust adjusts to the reduced load.
  • Mantle Convection:
    • Variations in mantle convection can affect the flow and pressure of the asthenosphere, influencing isostatic balance.